The present invention relates to frequency sources and, in particular, to sources utilizing a direct digital synthesis circuit.
Modern microwave frequency synthesizers in the general laboratory instrument class typically employ one of two architectures for achieving fine frequency resolution: fractional-N (Frac-N) or direct digital synthesis (DDS).
Referring to FIG. 1, a basic Frac-N synthesizer is shown. A reference frequency fref is applied to a phase-locked loop (PLL) having a fractional-N frequency divider in the feedback path. In the PLL, the fractional-N frequency divider allows the reference frequency to be multiplied by a value N·f that may include, not only an integer, but also, a fractional portion. This fractional capability is implemented by quickly switching between integer divisor values, resulting in an “average” frequency equivalent to a fractional division. With careful attention to design, Frac-N microwave synthesizers have been shown to provide excellent performance in terms of low spurious and phase noise content. However, such devices tend to be expensive and/or proprietary.
DDS circuits, as the name implies, directly generate a frequency. At their heart is an accumulator that accumulates a digital increment that conceptually corresponds to a phase increment. This phase increment is continually added to the sum of the previous phase increments. This phase value is used to address a sine-value table. The sine-value is applied to a digital to analog converter, which ideally would produce a frequency output of the DDS that is a pure sinusoid.
Currently, DDS synthesizers suffer from two undesirable performance characteristics. DDS generates relatively high levels of spurious signals in addition to the single desired sinusoid and it is capable of achieving only a moderate level of phase noise performance. A straight-forward application of DDS in a general laboratory instrument synthesizer will result in uncompetitive performance. However, DDS synthesizers are readily available and relatively inexpensive. For example, the company Analog Devices produces and sells a 2.7 GHz DDS synthesizer integrated circuit, model AD9956, that offers a 48-bit tuning word providing very fine resolution frequency generation.